chore(v2): add migration guardrails for backend purity and invoke audit

cx

scripts/check-v2-backend-purity.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+TARGET_FILE="$ROOT_DIR/src-tauri/src/commands_v2.rs"
+
+if [[ ! -f "$TARGET_FILE" ]]; then
+  echo "ERROR: File not found: $TARGET_FILE" >&2
+  exit 2
+fi
+
+PATTERN='crate::commands::|crate::library::commands::|crate::cast::[^[:space:]]*::commands::|crate::offline_cache::commands::'
+
+if rg -n "$PATTERN" "$TARGET_FILE"; then
+  echo
+  echo "FAIL: Forbidden legacy delegation found in commands_v2.rs"
+  exit 1
+fi
+
+echo "OK: commands_v2.rs backend purity check passed"